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A new bug disguising itself as a technical support website is targeting Firefox users on Windows and macOS.

According to a new report from Ars Technica, Firefox users on both Windows and macOS are being targeted by malicious sites that display a fake warning message and then completely lock up the browser. Hackers are taking advantage of a bug in Mozilla’s web browser to tamper with the software and render it unusable without the need for user interaction.

Malwarebytes’ Jérôme Segura warned that specially-crafted JavaScript is being used to exploit the vulnerability. The problem reportedly disguises itself as a fake technical support website, which then warns the visitor that they are using a pirated copy of Windows.

The warning message, which appears without any apparent user interaction on a website, reads:

Please stop and do not close the PC… The registry key of your computer is locked. Why did we block your computer? The Windows registry key is illegal. The Windows desktop is using pirated software. The Window desktop sends viruses over the Internet. This Windows desktop is hacked. We block this computer for your safety.

The message then advises the person to call a toll-free number in the next five minutes or face having the computer disabled.

“The only way to close the window to is to force-close the entire browser using either the Windows task manager or the Force Close function in macOS,” explains Ars Technica. “Even then, Firefox will reopen previously open tabs, resulting in an endless loop. To resolve the problem, users must force-close Firefox and then, immediately upon restarting it, quickly close the tab of the scammer site before it has time to load.”

Segura reported the bug to Mozilla and has since received word Mozilla is actively working on a fix. Firefox representatives couldn’t immediately provide information on the status of the bug.